1. Field of the Invention:
This invention relates to zoom lens focusing methods and more particularly to a focusing method using a lens group which is movable for zooming.
2. Description of the Prior Art:
Most conventional zoom lens focusing methods involve moving the frontmost lens component. Such a method is called the front focusing method. For example, there are four-component zoom lenses that focus at the first component, use the second component as the variator, the third component as the compensator and the fourth as an image forming component. Because this zoom lens configuration permits zooming without causing an earlier established in-focus condition to be broken, it has in the past found many uses in the various ratings of zoom lenses. Another system using the method is the so-called two-component zoom lens in which the first component of negative power and the second component of positive power form a zoom lens in which the components are moved in differential relation to effect zooming. When focusing, the first component is moved independently of the second one. After having been focused on an object at finite distances, zooming causes the sharpness of the image to change, more or less. This range of sharpness change is, however, so narrow as to constitute no problem in practical use. Therefore, two-component zoom lenses are popular along the above-mentioned four-component zoom lenses.
Nevertheless, the front focusing four-component zoom lens method has the following drawbacks:
(1) When focusing to shorter object distances, the front component is necessarily moved far ahead. For this purpose there is need to increase the diameter of the lens with a corresponding increase in the weight of the entire system.
(2) With the object at the closest distance, the height of incidence of the extra-axial ray on the lens surface becomes particularly high in wide angle positions, so that a rapid change in image aberrations is introduced.
(3) The foregoing two reasons prevent the focusing range from being extended very much towards the short side, so that for closeup photography a macro mechanism or other suitable supplementary means has to be used.
(4) The focusing lens moving block is positioned at the front of the lens mounting, so that the focusing lens is apt to change its position when the apparatus is subject to an outside its position when the apparatus is subject to outside shock.
(5) In an auto-focus camera the relatively heavy focusing lens and its operating mechanism make it difficult to maintain manageable proportions.
(6) The total movement of the focusing lens component is too great to avoid an over-load on the driving system in the auto-focus camera.
To eliminate the above-described drawbacks, it is necessary to employ some other focusing method than the front focusing one. Up to date, various proposals have been made.
Another focusing method for the above-described four component zoom lens is provided by moving the third lens component, which functions as the compensator, when zooming.
In this circumstance, with the third component, when moved to effect focusing, the required amount of axial movement of that component from a focusing position for an infinitely distant object, differs despite the same object distance, depending upon the zooming position.
To provide for focusing throughout the zoom components, or some component other than the first component gives the focusing movement a dependency on the zooming position. Therefore, focusing followed by zooming will break the in-focus condition, thus necessitating re-focusing. In order to compensate for the image shift resulting from focusing automatically during zooming, and to control the movement of the third component in accordance with both of the object distance and the focal length of the entire system, the operating mechanism must be provided with a cam of extremely complicated structure. It is, therefore, very difficult to manufacture mechanical mountings for such zoom lens economically with modern cam cutting techniques.
Zoom lenses having three components movable for zooming are described in U.S. Pat. Nos. 3,970,366 and 4,135,786 and Japanese Pat. Nos. Sho 39-6127 and 39-6432. With regard to the focusing through one of the zoom components, attention is invited to Japanese Laid-Open Pat. No. Sho 56-165107.